The definition of the layout and components that are served for requests under each device type.
The screenshot above shows the Presentation Details for a page. In this case, you'll note that we are using the Main layout, with various components for banners, menus, and display of terms. An item can exist in Sitecore without having presentation details (examples include metadata, settings, content which flows into components). Presentation Details can also be applied to a template standard values, meaning that any new item created, inherits those settings automatically.